Method for calculating bearing capacity of middle node assembly type concrete bridge column after being subjected to fire reinforcement

Abstract

The invention discloses a method for calculating the bearing capacity of a middle node assembly type concrete bridge column after fire reinforcement, and relates to the technical field of fire damage bridge column reinforcement, the calculation method is deduced on the basis of a reinforcement mode provided by a fire test, and the calculation method comprises the following steps: (1) material performance reduction calculation; (2) performing material performance recovery calculation after reinforcement; (3) deducing a test bending moment value after reinforcement; and (4) calculating the bending moment value after reinforcement. In practical engineering application, the residual bearing capacity of the middle node assembly type concrete bridge column after fire can be evaluated through a calculation formula provided by the invention, different reinforcing modes are selected according to damage conditions, and the bearing capacity of the reinforced structure is predicted and analyzed; and theoretical data support is provided for normal use of the fire damage bridge structure after reinforcement and repair.

Description

Technical Field

[0001] The present invention relates to the technical field of strengthening of fire-damaged bridge columns, and particularly to a method for calculating the bearing capacity of a fabricated concrete bridge column with a middle joint after fire strengthening. Background Art

[0002] At present, fabricated structures are widely used in bridge construction. Most of the existing research on fire resistance and strengthening is carried out based on cast-in-place concrete structures, and there is less research on fabricated concrete structures, lacking guidance for the application of fire repair technology of fabricated concrete structures in actual engineering applications. The research on the calculation and derivation of the residual bearing capacity of fabricated concrete bridge columns after fire, a complete set of mature strengthening processes, and the corresponding bearing capacity recovery calculation formulas is blank. Summary of the Invention

[0003] The present invention provides a method for calculating the bearing capacity of a fabricated concrete bridge column with a middle joint after fire strengthening, aiming to solve the problem in the prior art that "the research on the calculation and derivation of the residual bearing capacity of fabricated concrete bridge columns after fire, a complete set of mature strengthening processes, and the corresponding bearing capacity recovery calculation formulas is blank".

[0004] To achieve the above object, the technical solution of the present invention is as follows: A method for calculating the bearing capacity of a fabricated concrete bridge column with a middle joint after fire strengthening, the calculation method is derived based on the strengthening method provided by the fire test, and includes the following steps: (1) Calculation of reduction of material properties; (2) Calculation of recovery of material properties after strengthening; (3) Derivation of the test bending moment value after strengthening; (4) Calculation of the bending moment value after strengthening.

[0005] Preferably, the step (1) includes: for the strength of steel bars and concrete after fire, reduction coefficients are respectively selected for reduction; according to the determination of the compressive strength of concrete after high temperature in the literature [Wu Bo. Mechanical properties of reinforced concrete structures after fire [M]. Beijing: Science Press, 2003], the calculation formula (1) proposed by its highest overheating temperature is: ; (1); In formula (1), is the design value of the axial compressive strength of concrete, T is the highest temperature experienced by the concrete after damage. From the temperature rise curve obtained from the fire test, it is known that the highest temperature experienced by the concrete exceeds 200 °C, so is taken; The reduction of steel bars adopts the high-temperature steel bar stress reduction coefficient recommended in the literature [Wu Bo. Mechanical properties of reinforced concrete structures after fire [M]. Beijing: Science Press, 2003], and the expression (2) is: (2); Take .

[0006] Preferably, the step (2) includes: The strengthening method is: using A50 grouting material to replace the severely fire-damaged protective layer concrete of 30 mm on the outside of the specimen, and then strengthening the bridge column by wrapping carbon fiber strips; Based on the formula for calculating the influence of externally wrapped carbon fiber reinforcement on concrete strength proposed in the literature [Zhao Tong, Xie Jian, Dai Ziqiang. Experimental study on the complete stress-strain curve of concrete confined by carbon fiber cloth [J]. Building Structure, 2000, (07): 40-43+64. DOI: 10.19701 / j.jzjg.2000.07.009], the following formulas (3)-(5) are proposed for the precast concrete column with CFRP strips wrapped to strengthen the middle joint: (3); (4); (5); In formulas (3)-(5), is the peak stress of the confined concrete, is the peak stress of the unconfined concrete, is the fiber-containing characteristic value, is the volume ratio of the carbon fiber cloth to the confined concrete, is the reduced tensile strength of the carbon fiber cloth, is the cube compressive strength of the concrete, are the width and height of the cross-section of the concrete column respectively, are the width of the carbon fiber cloth and the strip spacing respectively, are the number of layers and thickness of the carbon fiber cloth respectively.

[0007] Preferably, the step (3) includes: During the pseudo-static test of the fire test, the lower end of the bridge column specimen is completely fixed, the upper part of the specimen is tightly connected to the MTS loading beam, and the force analysis of the specimen is simplified. For a specimen with a fixed support at one end and a sliding support at the other end, the bending moments at both ends are: (6); In formula (6), and are the bending moments at both ends of the specimen; is the horizontal load borne by the specimen; is the net length of the specimen.

[0008] Preferably, step (4) includes: based on the flexural bearing capacity calculation formula given in the Code for Design of Concrete Structures [GB 50010-2010. Code for Design of Concrete Structures, [S]. Beijing: China Architecture & Building Press, 2015.], the following formula is used to calculate the flexural bearing capacity of the precast concrete bridge column strengthened with CFRP after high temperature Calculation formula: (7); In formula (7), is the concrete compression coefficient. When the concrete strength grade does not exceed C50, is taken as 1.0; is the design value of the axial compressive strength of concrete; is the height of the concrete compression zone; is the effective height of the section; is the design value of the compressive strength of steel bars; is the cross-sectional area of the compressive steel bars; is the distance from the resultant force action point of the longitudinal compressive steel bars to the edge of the compression zone; is the eccentricity adjustment coefficient of the component end section. When it is less than 0.7, 0.7 is taken; is the moment amplification coefficient.

[0009] Beneficial effects of the calculation method for the bearing capacity of the precast concrete bridge column with a middle joint after fire strengthening in the present invention: In practical engineering applications, the residual bearing capacity of the precast concrete bridge column with a middle joint after fire can be evaluated by the calculation formula proposed in the present invention. Different strengthening methods can be selected according to the damage situation, the bearing capacity of the strengthened structure can be predicted and analyzed, and theoretical data support can be provided for the normal use of the fire-damaged bridge structure after strengthening and repair. Brief Description of the Drawings

[0010] Figure 1 is the limited-time fire test diagram of the present invention.

[0011] Figure 2 is the flow chart of the specimen strengthening of the present invention.

[0012] Figure 3 is the on-site diagram of the pseudo-static test of the present invention.

[0013] Figure 4 is the data diagram of the pseudo-static test of the present invention.

[0014] Figure 5 is the lateral bearing capacity curve diagram of the specimen after high temperature.

[0015] Figure 6 is the lateral bearing capacity curve diagram of the strengthened specimen (a is the specimen exposed to fire for 60 minutes; b is the specimen exposed to fire for 120 minutes).

[0016] Figure 7 It is a simplified diagram of the force analysis of the pseudo-static test. Specific implementation manners

[0017] The following description details the embodiments of the present invention in a step-by-step manner. This description is only for the preferred embodiments of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included within the protection scope of the present invention.

[0018] In the description of the present invention, it should be noted that the orientation or positional relationships indicated by the terms "upper", "lower", "left", "right", "top", "bottom", "inner", "outer", etc. are based on the orientation or positional relationships shown in the drawings, and are only for describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, as well as a specific orientation structure and operation. Therefore, it should not be construed as a limitation to the present invention.

[0019] Example 1: Fire test: A total of 5 identical precast concrete bridge columns were designed and fabricated, including 1 control column and 4 test columns; the test was divided into 3 stages: i) limited-time fire damage; ii) reinforcement and repair of damaged specimens; iii) cyclic lateral load under constant axial load; the 4 test columns were divided into two groups and subjected to limited-time fire tests for 60 and 120 minutes respectively, and 1 specimen from each group was selected for reinforcement; finally, the 5 precast columns were subjected to pseudo-static tests under cyclic lateral loads to evaluate the reinforcement effect of the specimens. The detailed working conditions of the specimens are shown in Table 1: Table 1 Test working condition table: ; Note: In the specimen number, ZZ represents precast concrete bridge column, 60 represents 60 minutes of fire exposure, and J represents reinforcement after fire. S1. Simulate fire high-temperature damage: The limited-time fire test was carried out in the disaster prevention and mitigation laboratory of Qingdao University of Technology. The vertical fire furnace had a size of 3000×3000×3000 mm and could achieve uniform fire exposure on all four sides. By controlling the spout, the furnace temperature was made to conform to the ISO834 standard heating curve; 5 K-type thermocouples were arranged at the grouting sleeve section in the column to measure the whole process change of the temperature during the heating and cooling stages of the specimen; the test applied a constant axial pressure through a hydraulic jack; to meet the actual engineering stress conditions of fixed connections at both ends, a pressure beam was used to fix the column pier on the bottom pier, and the steel plate embedded in the column head was connected to the top jack by bolts; to ensure that the column pier and the pressure beam were not affected by high temperature and there was no flame overflowing from the furnace cover at the top, rock wool was used to wrap and cover both the column top and the column pier. The details of the fire test are as Figure 1 shown; S2. Reinforcement and repair of high-temperature damaged specimens: As Figure 2As shown in the figure, the reinforcement of two precast concrete bridge columns with mid-span joints exposed to fire for 60 and 120 minutes is carried out in the following three steps: 1) Chisel off the 30-mm-thick protective layer concrete on the outer side of the specimen. After cleaning the specimen, formwork is carried out, and A50 grouting material is used for grouting repair and cured under natural conditions; 2) After 28 days of natural curing, chamfer the specimen to prevent stress concentration at the column corners after carbon fiber wrapping; 3) For the specimens exposed to fire for 60 and 120 minutes, single-layer and double-layer CFRP strips are selected and wrapped and pasted with epoxy resin. The strip width is 100 mm, the spacing is 100 mm, and the front and rear lap is 150 mm; S3. Quasi-static test: After the specimens are damaged by fire, the material strength may recover slowly, and the probability of an earthquake occurring immediately after a fire is small. Therefore, a quasi-static test is carried out one month after the limited-time fire damage to evaluate the influence of fire damage on precast concrete bridge columns; in the test, a computer-controlled MTS loading system is used to apply a horizontal cyclic load, and a hydraulic jack is used to apply a constant axial load with an axial compression ratio of 0.2; the embedded steel plate at the top of the column is connected to the four-link steel beam, and the column bottom is fixed to the ground through a pressure beam and bolts; six displacement sensors are installed to measure the displacement of the column body and the horizontal direction; The quasi-static test is carried out with displacement control for static two-way cyclic loading. The loading process is divided into three stages. The first stage is 0 - 15 mm, the second stage is 15 - 30 mm, and the third stage is from 30 mm to failure; different displacement increments (Δ / L) are adopted in each stage, corresponding to 1 / 600, 1 / 300, and 1 / 200 respectively, where "Δ" represents the horizontal displacement and "L" is the net length between the horizontal load application point and the contact surface at the bottom of the column; the cycle is repeated three times at each displacement level. When the load drops below 85% of the ultimate bearing capacity, the specimen is considered to have failed and the test stops; the detailed test information is as Figure 3 、 Figure 4 shown; It can be seen from Figure 6 that the combined reinforcement of CFRP and grouting material significantly improves the lateral bearing capacity of precast concrete bridge columns damaged by fire, even exceeding that of specimens without fire exposure. Such a reinforcement method has a good effect on the recovery of the lateral bearing capacity of specimens.

[0020] Example 2: A method for calculating the bearing capacity of a precast concrete bridge column with a mid-span joint after fire reinforcement. The calculation method is derived based on the reinforcement method provided by the fire test and includes the following steps: (1) Calculation of material property reduction; (2) Calculation of material property recovery after reinforcement; (3) Deduction of the test moment value after reinforcement; (4) Calculation of the moment value after reinforcement.

[0021] Example 3: Based on Example 1, this example discloses that step (1) includes: The loss of bearing capacity after a fire is mainly caused by the degradation of material strength. Under the condition of the ISO834 standard heating curve, the maximum temperature experienced by the bridge column specimen is related to the fire exposure time. Therefore, for the strength of steel bars and concrete after a fire, reduction coefficients are selected for reduction; According to the determination of the compressive strength of concrete after high temperature in the literature [Wu Bo. Mechanical Properties of Reinforced Concrete Structures after Fire [M]. Beijing: Science Press, 2003], the calculation formula (1) is proposed based on its maximum overheating temperature: : (1); In formula (1), is the design value of the axial compressive strength of concrete, T is the maximum temperature experienced by the concrete after damage. From the temperature rise curve obtained from the fire test, it is known that the maximum temperature experienced by the concrete exceeds 200 °C, so is taken; For the reduction of steel bars, the stress reduction coefficient of steel bars after high temperature recommended in the literature [Wu Bo. Mechanical Properties of Reinforced Concrete Structures after Fire [M]. Beijing: Science Press, 2003] is adopted. The expression (2) is: (2); Take .

[0022] Example 4: Based on Examples 1 - 3, this example discloses that step (2) includes: The reinforcement method is as follows: Use A50 grouting material to replace the severely fire-damaged 30-mm-thick protective layer concrete on the outside of the specimen, increasing the roughness of the contact surface. There is a good bonding force between the grouting material and the core concrete. The new and old concretes work together as a whole, improving the overall strength of the specimen concrete; Then, externally wrap carbon fiber strips around the bridge column. This method has a working mechanism similar to that of stirrups. When the concrete column is subjected to an axial force, the concrete will expand laterally and be passively constrained by the CFRP, which is equivalent to applying an inward compressive stress to the core concrete. The action and reaction forces between the carbon fiber cloth and the internally wrapped concrete make the concrete in a triaxial compressive state, enhancing the strength of the concrete column; Based on the formula for the influence of externally wrapping carbon fiber reinforcement on concrete strength proposed in the literature [Zhao Tong, Xie Jian, Dai Ziqiang. Experimental Study on the Stress-Strain Complete Curve of Concrete Confined by Carbon Fiber Cloth [J]. Building Structure, 2000, (07): 40 - 43 + 64. DOI: 10.19701 / j.jzjg.2000.07.009], the following formulas (3) - (5) are proposed for the CFRP strip-wrapped and reinforced middle-node precast concrete column: (3); (4); (5); In formulas (3)-(5), is the peak stress of the confined concrete, is the peak stress of the unconfined concrete, is the fiber-containing characteristic value, is the volume ratio of the carbon fiber cloth to the confined concrete, is the reduced tensile strength of the carbon fiber cloth, is the cube compressive strength of the concrete, are the width and height of the cross-section of the concrete column, respectively, are the width of the carbon fiber cloth and the strip spacing, respectively, are the number of layers and thickness of the carbon fiber cloth, respectively.

[0023] Example 5: Based on Examples 1-4, this example discloses that: The said step (3) includes: During the pseudo-static test of the fire test, the lower end of the bridge column specimen is completely fixed, the upper part of the specimen is closely connected to the MTS loading beam, and the force analysis of the specimen is simplified as Figure 7 shown; For a specimen with a fixed support at one end and a sliding support at the other end, the bending moments at both ends are: (6); In formula (6), and are the bending moments at both ends of the specimen; is the horizontal load borne by the specimen; is the net length of the specimen.

[0024] Example 6: Based on Examples 1-5, this example discloses that: The said step (4) includes: Based on the flexural bearing capacity calculation formula given in the specification [GB 50010-2010. Code for Design of Concrete Structures, [S]. Beijing: China Architecture & Building Press, 2015.], the following formula is used to calculate the flexural bearing capacity of the CFRP-strengthened precast concrete bridge column after high temperature : (7); In formula (7), is the concrete compression coefficient. When the concrete strength grade does not exceed C50, is taken as 1.0; is the design value of the axial compressive strength of the concrete; is the height of the concrete compression zone; is the effective height of the cross-section; is the design value of the compressive strength of the steel bar; is the cross-sectional area of the compression steel bar; is the distance from the resultant force action point of the longitudinal compression steel bar to the edge of the compression zone; is the eccentricity adjustment coefficient of the end section of the component, and when it is less than 0.7, it takes 0.7; is the bending moment amplification coefficient; The calculated bearing capacity values and test values obtained from the above formula are summarized in Table 2 as follows: Table 2 Comparison of calculated values and test values: ; By comparing the calculation results with the test results, it can be seen that the calculation results obtained by the calculation formula used in the present invention are in good agreement with the test results, generally in the range of 75% - 96%; the data used in the formula calculation are design values and the reduction value of high-temperature damage is relatively conservative, improving the safety redundancy, so the calculated values obtained by using the calculation formula proposed in the present invention are lower than the test values.

Claims

1. The calculation method of the bearing capacity of the middle node prefabricated concrete bridge column after fire reinforcement is characterized by: The calculation method is derived based on the reinforcement method provided by the fire test, evaluates the residual bearing capacity of the prefabricated concrete bridge column at the middle node after fire, selects different reinforcement methods according to the damage situation, predicts and analyzes the bearing capacity of the structure after reinforcement, and provides an evaluation of the normal use of the fire-damaged bridge structure after reinforcement and repair; it includes the following steps: (1) calculation of material performance reduction; (2) calculation of material performance recovery after reinforcement; (3) derivation of the test bending moment value after reinforcement; (4) calculation of the bending moment value after reinforcement.

2. The method for calculating the bearing capacity of the middle node prefabricated concrete bridge column after fire reinforcement as claimed in claim 1 is characterized by: The step (1) comprises: selecting reduction factors for the strength of steel bars and concrete after fire respectively. According to the literature on the compressive strength of concrete after high temperature The determination of the maximum overfire temperature is based on the calculation formula (1): （1）； In formula (1), is the design value of concrete axial compressive strength, T The maximum temperature of concrete after damage. The temperature rise curve obtained from the fire test shows that the maximum temperature of concrete exceeds 200℃, so ; The steel bar reduction adopts the steel bar stress reduction coefficient after high temperature recommended in the literature, and the expression (2) is: （2）； Pick .

3. The method for calculating the bearing capacity of the middle node prefabricated concrete bridge column after fire reinforcement as claimed in claim 2 is characterized by: The step (2) comprises: the reinforcement method is: using A50 grouting material to replace the 30mm protective layer of concrete on the outer side of the test piece that is severely damaged by fire, and then wrapping the bridge column with carbon fiber strips for reinforcement; Based on the calculation formula for the effect of carbon fiber reinforcement on concrete strength proposed in the literature, the following formulas (3)-(5) are proposed for the prefabricated concrete column with central node reinforced by CFRP strip wrapping: （3）； （4）； （5）； In formulas (3)-(5), is the peak stress of the confined concrete, is the peak stress of unconfined concrete, is the fiber content characteristic value, is the volume ratio of carbon fiber cloth to confined concrete, is the reduced tensile strength of carbon fiber cloth, is the compressive strength of concrete cube, are the width and height of the concrete column section, are the carbon fiber cloth width and strip spacing, are the number and thickness of carbon fiber cloth layers respectively.

4. The method for calculating the bearing capacity of the middle node prefabricated concrete bridge column after fire reinforcement as claimed in claim 3 is characterized by: The step (3) includes: during the quasi-static test of the fire test, the lower end of the bridge column specimen is completely fixed, and the upper part of the specimen is tightly connected to the MTS loading beam, and the force analysis of the specimen is simplified. For a specimen with a fixed support at one end and a sliding support at the other end, the bending moment at both ends is: （6）； In formula (6), and is the bending moment at both ends of the specimen; The specimen is subjected to horizontal load; is the net length of the specimen.

5. The method for calculating the bearing capacity of the middle node prefabricated concrete bridge column after fire reinforcement as claimed in claim 4 is characterized by: The step (4) comprises: based on the bending bearing capacity calculation formula given in the specification, the bending bearing capacity of the CFRP reinforced high temperature prefabricated concrete bridge column is calculated using the following formula: The calculation formula is: （7）； In formula (7), is the concrete compression coefficient. When the concrete strength grade does not exceed C50, Take it as 1.0; is the design value of concrete axial compressive strength; Height of concrete compression zone; is the effective height of the section; is the design value of steel bar compressive strength; is the cross-sectional area of ​​the compressive reinforcement; It is the distance from the point of action of the resultant force of the longitudinal compression reinforcement to the edge of the compression zone; is the adjustment coefficient of the eccentricity of the end section of the component, and when it is less than 0.7, it is taken as 0.7; is the bending moment increase factor.

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